Critolaus of Phaselis was a Greek philosopher of the 2nd century BCE. He was a leading member of the Peripatetic school, which followed the teachings of Aristotle, and served as its head in Athens around the middle of that century. He is most historically notable for being part of the famous "Philosophers' Embassy" sent from Athens to Rome in 155 BCE. The delegation, which included prominent Stoic and Academic philosophers, aimed to appeal a political fine. Their speeches, which introduced Roman audiences to advanced Greek philosophical debate, caused a significant cultural stir.

None of Critolaus's own writings survive intact; they are known only through fragments and reports by later authors. He is said to have written treatises on topics such as the soul, ethics, and physics. According to modern scholars, his significance lies in his role as a major defender of Aristotelian thought during a period when the Peripatetic school was less influential. In ethics, he argued that virtue was of "incomparable" worth compared to other goods. In physics, he upheld the Aristotelian view of an eternal world against rival Stoic theories. Academics regard him as the most important head of the Peripatetic school between the 3rd and 1st centuries BCE, and his embassy to Rome marks a key moment in the transmission of Greek philosophy to the Roman world.